The field of this disclosure relates generally to a gas turbine engine and, more particularly, to a gas turbine engine having an adjustable flow path geometry.
Many known gas turbine engines have a ducted fan and a core arranged in flow communication with one another. The fan provides air to the core (a “core flow”) and to a bypass duct surrounding the core (a “bypass flow”). The core compresses the core flow and subsequently mixes it with fuel for igniting the mixture to generate a flow of combustion gas through a turbine. The combustion gas drives the turbine, which in turn drives the fan to generate the core flow and the bypass flow.
With the bypass flow being a source of thrust for the engine, some known fans have blades for which the pitch can be varied to facilitate controlling the thrust. In that regard, these fans can be configured such that the blades, at one pitch angle, generate an aftward directed bypass flow resulting in forward thrust, and, at another pitch angle, generate a forward directed bypass flow resulting in reverse thrust. However, in these known engines, the condition of the bypass flow is often less than optimal in both directions. As such, for gas turbine engines having variable pitch fans, it would be useful to improve the condition of the bypass flow.
A key metric in establishing commercial aircraft performance is the engine's propulsive efficiency. Propulsive efficiency is a measure of the amount of power the engine delivers to the aircraft as a fraction of the total amount of kinetic power the engine imparts into the air passing through it. To achieve a high propulsive efficiency, it is desirable for the engine to impart only a minor change in jet velocity to the air it acts upon. Because this yields only small amounts of thrust per unit airflow, the engine must act on proportionately larger quantities of air in order to make sufficient thrust. As the amount of air the engine (fan) acts on increases, the weight and drag of the engine also increases. At some point, the additional weight and drag accompanying the larger diameter fan begin to outweigh the benefit of improved propulsive efficiency.
It is therefore desirable to enable modifying a geometry of the engine gas flowpath to improve propulsive efficiency of the engine over a wide operational range while reducing the weight of engine components.